This invention relates to local area networks, and in particular, to the provisioning of power to terminals via the local area network.
Within the prior art, telephone switching systems such as PBXs have traditionally provided the power to telephone sets via a telephone link between the telephone set and the telephone switching system. The power supplied to the telephone sets has been at 48 volts. Local area networks on the other hand have not within the prior art provided power to devices connected to the LAN. These devices have been personal computers (PC), printers, etc. Devices such as PCs and printers receive their power from batteries or power supplies that plug in to a local AC power outlet. Within the prior art it is known to add telephone sets to a LAN by providing local power from an AC power outlet to the telephone set. However, this is not generally acceptable to customers. The problem of supplying power via the LAN (referred to as phantom power) is complicated because the LAN will have a mixture of telephone sets and other devices requiring phantom power and devices that do not require power from the LAN. Nor, is a device such as a PC capable of withstanding 48 volts of phantom power in its LAN connections. In addition, whereas within a four-pair LAN wiring system only two of the pairs are utilized for data and there is indeed a spare pair that could be used for power, it is common for all of the pairs to have approximately 75 ohms of terminating resistance placed across the unused pair so as to balance the pair and reduce noise induction. Further, in the field, there is no control over what will be plugged in to different connections of the LAN. So it is quite possible that a PC will be plugged in to the LAN and suffer damage. Conversely, it is possible that someone will plug in a legacy telephone set that can withstand the 48 volts but is totally incompatible with the operation of the LAN.
Within the prior art, it has been proposed that a device wishing to receive phantom power via the LAN provide a signature of a 25 Kxcexa9 resistor when initial power is applied via the LAN pairs. The 25 Kxcexa9 resistor may reside behind or in front of a polarity guard that comprises diodes and/or transistors. The polarity guard protects the telephone set from the possibility that voltage will be applied in the reverse direction. The polarity guard causes the value of the 25 Kxcexa9 resistor to vary due to temperature, the voltage drop variation caused by diodes and leakage of current by transistors. Further, because of the existence in many existing installations of LANs of the unused pair being terminated by 75 ohm resistors and the unused pair being a common choice for the phantom power the testing for the 25 Kxcexa9 resistor is quite complicated.
This invention is directed to solving these problems and other disadvantages of the prior art by determining that a slope of test values resulting two reference voltages being applied to a remote device and a reference circuit is within a predetermined range and applying full power to the remote device if the slope is within the predetermined range. Advantageously, the determination using the slope greatly reduces the effects of voltage drop variation caused by diodes and leakage of current by transistors in the remote device. The determination is performed by first applying the first reference voltage to both the remote device and reference circuit and storing the resulting two voltages. Next, the second reference voltage is applied to both the remote device and reference circuit and the resulting two voltages are stored. A first difference is calculated from the two voltages resulting from the testing of the remote device, and a second difference is calculated from the two voltages resulting from the testing of the reference circuit. The absolute difference between the first and second differences is calculated. Finally, the absolute difference is compared with the predetermined range.
Advantageously, before the determination is made, the remote device is tested to assure that the impedance of the remote device is within a second predefined range. The testing is performed by applying the first test voltage to both the remote device and reference circuit and calculating the expected voltages for the second predefined range from the test result obtained from the reference circuit.
Other and further aspects of the present invention will become apparent during the course of the following description and by reference to the accompanying drawing.